Cathode structure for magnetrons



March 1, 1949. w, FQRSBERGH, JR 2,463,372

CATHODE STRUCTURE FOR MAGNETRONS Filed Oct. 3, 1945 FIG/I. mr

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FIG.2 FIGS. FIGA. FIG.5. H66.

I3 2o I3 20 [I37 3 20 I3 I 405 I0\' 7 I an I 300 4o2 I2 302 I 400 502 E m I I4 I4 I aw 2| [El fi dl FIG.7. FIGB. NORMAL NORMAL OPERATING OPERATING POINT POINT STARTING POINT STARTING POINT TO ANOL'L: VULI'AG E E TO ANODE VOLTAGE L... MAGNETRON SPACE CURRENT QMAGNEI'IRON SPACE CURRENT 8MAGNHRONS HAVING gMA NErRoNs HAVING I cusTOMARY CYLINDRICAL I-CUSTOMARY CYLINDRICAL CATHODE WITHOUT RADIALLY SCATHODE WITH RADIALLY U ENLARGED CATHODE PORTIONS ENLARGED CATHODE PORTIONS IN V EN TOR.

PETER W. FORSBERGH JR.

Patented Mar. 1, 1949 UNITED PAT E'NT OFFICE CATHODE-STRUCTURE FOR MAGNETRONS Peter'W. Forsbergh,-Jr., Cambridge, Mass., as-

signor, by rnesne-assignments, to the United States of America as represented by the. Secreetary'oi War ApplicationOctober 3, 1945, SerialNo. 620,126

8 Claims. (Cl. 25027.5)

The invention relates to improvements; in elec-- trical apparatus and more particularly to improvements in magnetrons.

In thepural cavit magnetron, the anode of which isconstructedin several segments and disposed about a central cathode, several modes of radio frequency oscillations are possible. Certain In the accompanying drawings forming a part of this specification and-in Which-like numerals are employed to designate like parts throughout the same:

Fig. 1 is acut-away isometric View of a portion of a magnetronembodying the invention;

Figs. 2 throughfi are side views of: cathodes embodying the invention;

Fig. 7 is a voltage vs. current characteristic curve-for a magnetronwhich does not embody.

the invention; and

Fig. 8 is a voltage vs. current characteristic curve for amagnetron embodying the invention:

Referring to Fig. 1, the numeral-l0 designates the main cathode which is cylindrical in construction. At the op osite endsof the rnain cathode ID- are radially expanded end portions H and I2. which-are also illustrated in Fig. 2. In the preferred embodiment shown; the radially expanded end portions II and I2 are shownas larger diameter steps or rings on the'main cathode it, but they may be tapered sections, 300 combined with ring sections 302;discs 40B; 402, Fig. 4, or equivalentmeans for creating a minor electron source radially closer to the anode segment l5 than is the anode-facing surface of the main cathode IE. Although the radially expanded sections or rings are preferably located at the ends of the cathode,

inthe. middle, and two rings. E05 and 506at the bottom.

Referring to Fig.2, the main cathode H} and the radially expanded end portions l I and I2 may be coated with electron emitting material, such as a mixture of barium and strontiumcarbonates, prior, to the exhaust of thev tube. However, the radially expanded end portions ii and !2' may be left uncoated if desired.

Members i3 and I4 designate end shields thermally isolated from the. cathode structure IE! for the purpose of maintaining the end shields i3 and I4 relatively cool with respect to the cathode, and electrically connected tothe cathode it).

by conductors Hand 2 1. Members 22 and 23 and pole piecesM and zii. are used to seal the magnetron. Permanent magnet 25 furnishes the necessary magnetic fluid. The end'shields l3 and i i tend to minimize undesirable electron leakage to lids 22, 23 andpole pieces 2t and 25.

The operation of the improved magnetron cathode structure is as follows:

The. essential feature of the invention is to provide a minor source of electrons (hereshown as radially expanded or radiall enlarged portions), radially closer to the anode i5jthan is the remaining coated cylindrical surface of the main cathode H), which acts as the major source of electrons.

The radially enlarged portions serve to decrease moding and thus to stabilize the operation of the magnetron. The following hypothesis is advanced to explain the. operation of these cathode structures:

In eneral, the desired mode. of operationfor themagnetron isthat in'which alternate anode.

" such. as H and..l2,.is,probably-toraise, the noise level from whichthe radio frequency buildupzoi thepi-mode start, and hence lower the starting time. The radially enlarged end portions (or the equivalent means'of; creating; a. minor electroni source radially closer to the anodepsegments 15+ than in themain cathode Ill). may be madeelectron ,emissiveyeither by beingcoated .with electron emitting material prior to the evacuation of the magnetron, or when not coated with electron emissive material, the radially enlarged end sections become electron emissive due to contamination with the emissive material which is evaporated, sputtered or surface-diffused from the main cathode I during the usual steps of processing the magnetron.

The effect of this invention on the voltage vs. current characteristic curves of the magnetron is illustrated in Figs. 7 and 8.

a. Magnetrons having the customary cylindrical cathode (that is, without radially enlarged cathode section or step), Fig. 7, and

b. Magnetrons having a cylindrical cathode with radially enlarged sections (that is, having a step cathode), Fig. 8.

Comparison of these two curves reveals the fact that the Starting point in Fig. 8 is reached at a much lower voltage than the same point in Fig. 7. Therefore with the rate of voltage change remaining constant in both cases, the magnetron having stepped cathode will reach the operating point quicker without drifting into the other than pi-mode oscillations.

Heretofore, the chief known method of improving the mode stability of magnetrons was to obtain cathodes of high primary emission. This involved close supervision of the cathode manufacture and extremely careful control of impurities in the cathode materials.

Excellent magnetrons have been made, even though cathodes of low primary emission were used, with the present improved cathode structure having radially enlarged end sections. The mode stability has been improved appreciably and the efliciency is essentially unimpaired. This magnetron cathode structure may be used to reduce the stringenecy of manufacture of cathode emission materials.

While there has been described what is at present considered to be a preferred embodiment of this invention, it will be obvious that various changes and modifications may be made therein without departin from the scope of the inVention as set forth in the appended claims.

What is claimed is:

l. A magnetron comprising, a main cylindrical cathode having an electron emissive layer constituting a major source of electrons, an anode concentrically disposed With respect to said cathode, at least one radially enlarged portion on said emissive layer, said portion being thermally at substantially the same thermal level as said cathode but mechanically nearer to said anode, said portion constituting a minor source of electrons, and disc-shaped metallic end shields disposed at the opposite ends of said cathode and thermally insulated therefrom, said end shields being electrically connected to said cathode, the diameter of said end shields being greater than the diameter of said enlarged portion.

2. A magnetron having a cathode including a main cylindrical part and a plurality of radially enlarged ring portions, said ring portions constituting direct metallic extensions of said metallic cylindrical part, said main cylindrical part and said radially enlarged portions all being coated with electron emissive material, whereby said cathode structure serves to minimize moding of the magnetron.

3. A magnetron having a cathode structure including a main cylindrical cathode with an electron emissive layer on the outer surface of said cathode, three radially enlarged cathode portions on the outer surface of said cathode for minimizing moding in said magnetron, said electron-emissive layer also extending over said radially enlarged cathode portions, and metallic end shields thermally insulated from but electrically connected to said cathode.

4. A thermionic tube including an anode having a plurality of elements disposed along a cylindrical surface, an indirectly heated cathode concentrically disposed with respect to said anode, said cathode comprising a right metallic cylinder, an electron emissive layer on the entire surface of said cylinder facing said anode, said cathode having more than two axially spaced ring-shaped portions radially extending toward said anode beyond the remaining portion of said layer, said ring-shaped portions constituting direct, metallic radial extensions of said metallic cylinder and acting as the major source of electrons during the starting portion of the operating period of said tube.

5. A thermionic tube as defined in claim 4 in which said ring-shaped portions lie in the respective planes parallel to each other and perpendicular to the longitudinal axis of said cylinder.

6. A magnetron comprising an anode, a cylindrical cathode in-coaxial relationship with respect to said anode, said cathode including a hollow metallic cylinder, a radially displaced portion on and constituting a direct, metallic extension of said metallic cylinder, said portion being of smaller axial length than the remaining portion of said cathode and nearer to said anode than said remaining portion, and an electron emissive layer on the entire anode side of said cylinder whereby said radially displaced portion constitutes a minor source of electrons while the remaining portion of said cathode constitutes a major source of electrons.

7. A magnetron as defined in claim 6 which also includes metallic end shields at the opposite ends of said cylinder, said end shields bein thermally insulated from but electrically connected to said cathode.

8. A magnetron including an anode, a cathode having a metallic cylinder in coaxial relationship with respect to said anode, a plurality of radially displaced ring-shaped portions on and constituting a direct metallic extension of said metallic cylinder, said ring-shaped portions being nearer to said anode than the remaining portion of said anode and having the combined area smaller than the remaining area of said cathode, and an electron emissive layer on the entire surface of said metallic cylinder facing said anode.

PETER W. FORSBERGH, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,121,360 Malter et al. June 21, 1938 2,372,037 Watrous Mar. 20, 1945 2,400,770 Mouromtseif et a1. May 21, 1946 2,406,276 White Aug. 20, 1946 2,414,581 Buritz et al. Jan. 21, 1947 2,416,899 Brown Mar. 4, 1947 2,430,309 Spencer Nov. 4, 1947 

